The present invention relates to a method and a device for transmitting a video bitstream over networks. More specifically, the present invention relates to a method and a device for the transmission of a variable bit-rate compressed video bitstream over both constant and variable capacity networks.
Digitally compressed video is now widely used for one and two-way communication of visual information. This is largely facilitated by the existence of a number of international standards for compressed digital video, including MPEG-1, MPEG-2, and MPEG-4. These standards specify a particular bitstream syntax for representing visual information in a binary fashion. The compressed bitstream may then be communicated over a specific channel.
The bit-rate of a compressed video bitstream is a function of time. This function depends mainly on two factors. The first factor is the amount of information contained in the original video sequence at each time period that the compression algorithm analyzes. The second factor is the amount of data that is deemed insignificant and discarded by the compression algorithm. If the goal of the compression algorithm is to achieve constant-visual quality across the whole video sequence, the bit-rate of the resulting bitstream usually varies over time. However, the physical networks that are used to transmit these video bitstreams may either be of constant or variable capacity.
For transmission of compressed video sequences over constant capacity networks, such as circuit switched wireline or wireless phone systems, existing rate control mechanisms usually enforce the constant bitrate constraint and generate variable-quality bitstreams. This method has the side effect of producing poor quality compressed video segments for scenes with large motion while wasting transmission bandwidth for scenes with static objects.
The delivery of video over emerging variable capacity wireline and wireless communication systems entails a number of new hurdles to be crossed. One critical element of these new systems is that they provide a means for variable bit-rate communication. Whether packet-switched communication over a corporate LAN or variable rate CDMA wireless communication, this critical component of the communication system should be exploited to maximize the quality of delivered video. Existing rate control methodologies do not effectively exploit the variable and dynamically changing bandwidth characteristics of modern communication systems. Accordingly, the quality of encoded video is often unnecessarily poor in digital video delivery systems.
A need, therefore, exists for an improved method and device for transmitting a variable bit-rate compressed video bitstream over constant and variable capacity networks that overcome the problems associated with existing methods and devices.
The present invention addresses the problems of producing and delivering constant quality bitstreams over channels of constant or variable bit-rate capacity. The present invention may be integrated within any standardized video coding system, but the preferred embodiment is integrated within the MPEG-4 standard. The method and device relate to achieving constant quality over both constant capacity and variable capacity networks. For constant capacity networks, the method of the present invention re-distributes the coded bits in a constant-quality variable bit-rate bitstream to construct a constant bit-rate bitstream. The re-distribution of the coded bits are done in an MPEG-4 standard-compliant fashion so that the resulting bitstream may be decoded by any MPEG-4-compliant video decoder as a variable quality bitstream. However, a matching decoder at the receiver, consistent with the present invention, may be used to restore the original variable bit-rate bitstream so that the constant-quality criterion is maintained.
For variable capacity networks, the method and device relate to achieving constant quality as described by a metric that incorporates both spatial and temporal visual quality. The method incorporates user, network and channel parameters in controlling the bit-rate while encoding video material. Network parameters including average packet loss rate, average packet size, number of users on the network and statistics of individual users (mean and standard deviation of the bit-rate of connected sessions) are used to choose and adapt the free parameters of the rate controller. The method and device allow feedback from the receiving terminal to influence the encoding parameters. These features provide for context-based rate control that may adapt to the activity and nature of the coded scene. The method and device may be used for real-time or off-line coding, and all automatic parameter computations may be overridden by the user during run-time.
To this end, in an embodiment, a method for controlling a digitally compressed video bitstream for achieving constant quality video delivery over a constant capacity channel is provided. The method comprises the steps of: providing an encoder; providing a digitally compressed video bitstream having frames; analyzing a quantity of bits used to compress each frame; and repositioning an excess set of bits associated with an original bitstream position in a first frame to a predetermined bitstream field associated with a second frame having a lesser number of bits than the first frame to create an altered bitstream.
In an embodiment, the altered bitstream is delivered at a constant rate over a constant capacity channel.
In an embodiment, the altered bitstream is parsed. The bits associated with the predetermined bitstream field associated with the excess set of bits are buffered and the excess bits are re-positioned into the original bitstream position.
In an embodiment, the bitstream is decoded to generate a constant quality video sequence.
In an embodiment, the excess set of bits associated with the original bitstream position is a set of enhancement layer bits.
In an embodiment, the bitstream is an MPEG standardized bitstream.
In an embodiment, the predetermined bitstream field is a user-defined data field.
In an embodiment, the bitstream has a variable bit-rate.
In an embodiment, the constant capacity channel has a bit budget for each frame and further wherein each frame has at most the number of bits allowed by the bit budget.
In another embodiment of the present invention, a method for controlling a digitally compressed video bitstream for achieving constant quality video delivery over a variable capacity channel is provided. The method comprises the steps of: providing a network; providing a bitstream; providing a user in the network; analyzing parameters based on preferences of the user; managing a temporal frame rate of the bitstream based on the parameters; changing a quantization of residual frames based on the parameters; providing a coding process; and splitting the coding process into a layered coding process based on some of the parameters.
In an embodiment, the parameters relate to quality of service preferences of the user.
In an embodiment, the parameters include at least one of temporal quality measures, spatial visual quality measures, network capacity, network availability, enhanced feature compliance and a number of users supported by the network.
In an embodiment, the enhanced feature compliance includes scalability.
In an embodiment, some of the parameters include a range of enhanced feature compliance supported by the network and a number of users in the network.
In an embodiment, a network manager is provided. The preferences of the user are communicated via a channel to the network manager.
In another embodiment of the present invention, a device for controlling a digitally compressed video bitstream for achieving constant quality video delivery over a constant capacity channel is provided. The device has an encoder for analyzing a digitally compressed video bitstream having frames to determine a quantity of bits used to compress each frame and further wherein the encoder repositions an excess set of bits associated with an original bitstream position in a first frame to a predetermined bitstream field associated with a second frame having a lesser number of bits than the first frame to create an altered bitstream.
In an embodiment, a decoder is provided for parsing the altered bitstream wherein the decoder buffers the bits associated with the predetermined bitstream field associated with the excess set of bits and further wherein the decoder repositions the excess set of bits into the original bitstream position.
In another embodiment of the present invention, a device is provided for controlling a digitally compressed video bitstream for achieving constant quality video delivery over a variable capacity channel. The device has an analyzer for analyzing a digitally compressed video bitstream wherein the analyzer analyzes parameters based on preferences of a user. A manager is provided for managing a temporal frame rate of the bitstream based on the parameters. Means is provided for changing a quantization of residual frames in the bitstream. A splitter is provided for splitting a coding process into a layered coding process based on some of the parameters.
In another embodiment, the parameters include at least one of temporal visual quality measures, spatial visual quality measures, network capacity, network availability, enhanced feature compliance, and a number of users supported by the network.
In an embodiment, some of the parameters that are used to split the coding process into the layered process include a range of enhanced feature compliance supported by a network and a number of users in the network.
It is, therefore, an advantage of the present invention to provide a method- and a device for transmitting a variable bit-rate compressed video bitstream over constant and variable networks.
Another advantage of the present invention is to provide a method and a device that may be integrated within any standardized video coding system.
Additionally, an advantage of the present invention is to provide a method and a device that achieve constant quality over both constant capacity and variable capacity networks.
A still further advantage of the present invention is to provide a method and a device that redistribute the variable coded bits in a bitstream to construct a constant bit-rate bitstream.
Moreover, an advantage of the present invention is to provide a method and a device that incorporate user, network and channel parameters in controlling the bit-rate while encoding video material for variable capacity networks.
A further advantage of the present invention is to provide a method and a device that allow feedback from the receiving terminal to influence the encoding parameters.
A still further advantage of the present invention is to provide a method and a device that provide for context-based rate control which can adapt to the activity and nature of the coded scene.
In addition, an advantage of the present invention is to provide a method and a device that may be used for real-time or off-line coding whereby all automatic parameter computations may be overridden by the user during run time.
Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings.